In recent years, there have been increasingly severe demands for the performance of silver halide light-sensitive materials for photographic use. Accordingly, there have been requirements for increased levels of storage stability and photographic properties such as sensitivity, fog and graininess. With the recent popularization of compact zoom cameras and so-called single-use cameras or films with lens, high sensitivity has become an essential feature of photographic light-sensitive materials.
Moreover, sophisticated cameras have permitted ordinary users to easily enjoy various advanced photographic techniques and have accordingly produced new demands for improved sensitivity and improved tone reproducibility under every set of exposure conditions.
Thus, various methods of improving silver halide light-sensitive materials are now under development. As a prior art means of improving the sensitivity of silver halide emulsion, mention may be made of the silver halide emulsion grains of the core/shell type with high inner iodide content characterized by multiple layer-structured grains, disclosed in Japanese Patent Publication Open to Public Inspection (hereinafter referred to as Japanese Patent O.P.I. Publication) No. 14331/1985. This method aims at improving the blue light absorbing efficiency while maintaining high developing activity by covering a low iodide phase (phase having a low silver iodide content; the same applies below), formed inside the grain, with a high iodide phase (phase having a silver iodide content higher than that of the low iodide phase; the same applies below).
However, this method is not expected to be effective on the visible light rays out of the specific absorption band of silver halide, i.e., red light and green light, though it serves to increase the absorption efficiency for the visible light rays in the specific absorption band, namely blue light rays.
It is a common practice to cause a dye called spectral sensitizer to adsorb on silver halide to make a color sensitive material sensitive to red light and green light, which are not absorbed by silver halide grains.
Spectral sensitizing dyes act to absorb the light in a particular wavelength band (sometimes specific absorption) which is not usually absorbed by silver halide and provide the resulting photoelectron for the silver halide. However, if the adsorption between spectral sensitizing dye and silver halide grains is weak, dye desorption may occur during storage of the light-sensitive material (this tendency increases under hot humid conditions), which in turn can degrade the sensitivity. Therefore, enhancing the adsorption between spectral sensitizing dye and silver halide grains not only improves the storage stability but also increases the effective adsorption amount of the sensitizing dye, and is considered to result in an improvement in the light absorption efficiency of the silver halide grains.
As a means of improving spectral sensitizing dye adsorbability and suppressing intrinsic desensitization, the silver iodide content in the grain surface is increased in some known methods. Japanese Patent O.P.I. Publication No. 183646/1989, for example, discloses a light-sensitive material which has high sensitivity and which is less liable to intrinsic desensitization, specifically core/shell type grains having a silver iodide content of not less than 6 mol % in the shell. It is stated therein, however, that the silver iodide content of the core is preferably not more than 5 mol %, more preferably not more than 3 mol % for accelerating the development. Also, the emulsions described in Examples are all comprise core/shell type grains with a low inner iodide content, i.e., this method is limited to core/shell type grains wherein the inner iodide content is low.
On the other hand, Japanese Patent O.P.I. Publication No. 12142/1990 discloses a light-sensitive material which has high sensitivity, which is less liable to intrinsic desensitization and which is less liable to pressure/stress fogging, specifically silver halide grains which have an outermost shell whose silver iodide content is higher than that of the core at not less than 6 mol %, at least one intermediate shell between the core and the outermost shell and an aspect ratio of lower than 8. As is obvious to those skilled in the art and as stated in the specification for that patent, grains having a high surface silver iodide content are undesirable for use as a photographic light-sensitive material for color negative films, since the progression of development is considerably retarded. This is because the iodide in the grain surface region suppresses development, since color development is of surface development.
However, the data on the evaluation of the sensitivity and intrinsic desensitization in Examples does not reflect the performance of the color light-sensitive material, since black-and-white development, which is hardly affected by development suppression by iodide, is used. In addition, in the color development, evaluation data was obtained for intrinsic sensitivity alone, since the spectral sensitizing dye was not adsorbed. Moreover, no comparison was made with core/shell type grains having a high inner iodide content in this case.
As stated above, none of the conventional color photographic light-sensitive materials incorporating an emulsion of core/shell type grains having a low inner iodide content offers satisfactory improvement in sensitivity or fog reduction.
Japanese Patent O.P.I. Publication No. 106745/1988 discloses a light-sensitive material which is excellent in spectral sensitizing property and which is not liable to performance deterioration under humid conditions, specifically core/shell type grains having a high inner iodide content and a surface silver iodide content of not less than 5 mol %. The specification for that patent describes a method of introducing silver iodide to the grain surface wherein fine silver iodide grains of not more than 0.1 .mu.m or fine silver halide grains having a high silver iodide content are added. However, the introduction of silver iodide to the grain surface in Examples is always achieved using the double jet method or an aqueous solution of potassium iodide. In addition, there is no description of a method of forming the core and shell using fine silver halide grains; in Examples, silver halide grains are prepared by the controlled double jet method.
This method does not offer a satisfactory effect, since the degrees of improvement in the sensitivity, color sensitizing property and storage stability are low.
When a silver halide photographic light-sensitive material is subjected to exposure at high intensity for a short time or at low intensity for a long time, the obtained image density is rarely constant even when the amounts of exposure are equal to each other. Such changes in sensitivity and tone depending on exposure intensity is referred to as the reciprocity law failure. The reciprocity law failure occurring in high intensity exposure relative to optimum exposure conditions is referred to as high intensity reciprocity law failure, and the reciprocity law failure occurring in low intensity exposure is referred to as low intensity reciprocity law failure.
In a light-sensitive material with a significant reciprocity law failure, the exposure time must be corrected according to the illuminance and light source. When the layers of a multiple layered color light-sensitive material have different degrees of reciprocity law failure, the obtained image shows color fluctuation according to exposure time.
To improve this reciprocity response, various methods of improving silver halide light-sensitive materials are under development. The prior art of improving the reciprocity response of silver halide emulsions is based mainly on silver halide grains doped with ions of metals primarily those belonging to the group VIII in the periodic table of elements. Japanese Patent O.P.I. Publication Nos. 184740/1988, 183647/1989 and 183655/1989, for example, disclose methods of improving the reciprocity response by doping with ruthenium and iridium ions, iron ion and rhodium ion, respectively.
However, these methods based on metal ion doping are not expected to have an effect on the low intensity reciprocity law failure, and its improving effect on the high intensity reciprocity law failure property is not satisfactory. Moreover, sensitivity reduction and increased fog pose other problems.
International Application No. 06831/1989 discloses a silver halide light-sensitive material which has high sensitivity and which is less liable to fogging, specifically reduction-sensitized silver halide grains wherein crystals were grown in the presence of fine silver halide grains. It is evident from the description of the objects and effect of the method in the specification, however, that this method does not meet the structural requirement of the present invention to have an improving effect on the reciprocity law failure property.
Also, Japanese Patent O.P.I. Publication No. 222939/1990 discloses a silver halide photographic light-sensitive material which has high sensitivity, especially in the spectrally-sensitizing range, and which is less liable to fogging, specifically silver halide grains containing not less than 5 mol % silver iodide on the grain surface which has been reduction sensitized during their growth. However, the silver halide grains described in Examples are core/shell type grains having a high inner iodide content wherein the silver iodide content of the shell has been increased to not less than 5 mol %, which are totally different from the silver halide grains of the present invention. In addition, this method does not offer an improvement in the reciprocity law failure property.
As stated above, there is no prior art method which offers high sensitivity and suppressed fog and which makes it possible to improve the reciprocity response.